Fluoromethyl hexafluoroisopropyl ether, known as “Sevoflurane”, is used as an inhalation anesthetic with good anesthetic performance. As a production method of fluoromethyl hexafluoroisopropyl ether, there is known a process of reacting formaldehyde (paraformaldehyde) with hexafluoroisopropyl alcohol in a liquid phase by the use of sulfuric acid as a dehydrating agent in hydrogen fluoride (see Patent Document 1). Further, there is proposed a process for obtaining fluoromethyl hexafluoroisopropyl ether by selective solvent extraction or distillation, in place of using a dehydrating agent, from a reaction mixture in which hexafluoroisopropyl alcohol and formaldehyde are in equilibrium in hydrogen fluoride (see Patent Document 2). It has been found that several percentages of fluoromethyl hexafluoroisopropyl ether can be obtained by mixing hydrogen fluoride with formaldehyde (trioxane), cooling the mixture, adding hexafluoroisopropanol to the mixture, and then, stirring the resulting mixture (see Patent Document 3). There is also proposed a process for producing fluoromethyl hexafluoroisopropyl ether by reacting high-purity bisfluoromethyl ether with hexafluoroisopropyl alcohol in the presence of sulfuric acid (see Patent Documents 4 and 5).